Oxygen transport from the lungs to body tissue can be monitored by measuring various physiological parameters. For example, oxygen saturation of arterial blood (SaO2) is a measure of the ratio of oxyhemoglobin (HbO2) concentration to the sum of HbO2 and deoxyhemoglobin (Hb) concentrations in the arterial blood. Because HbO2 is the major oxygen carrying component of blood, SaO2 is indicative of oxygen delivery to body tissues. As another example, oxygen saturation of venous blood (SvO2) is a similar measure of HbO2 and Hb concentrations in venous blood and is indicative of oxygen consumption by body tissues. Measurements of the concentrations of carboxyhemoglobin (HbCO) and methemoglobin (MetHb) are indicative of abnormal hemoglobin constituents that interfere with oxygen transport.
Pulse oximetry is a noninvasive, easy to use, inexpensive procedure for measuring the oxygen saturation level of arterial blood. Pulse oximeters perform a spectral analysis of the pulsatile component of arterial blood in order to determine oxygen saturation (SpaO2), which is an estimate of SaO2. A pulse oximetry system has a sensor and a monitor. The sensor has emitters that typically consist of a red light emitting diode (LED) and an infrared LED that project light through blood vessels and capillaries underneath a tissue site, such as a fingernail bed. A sensor also has a detector that typically is a photodiode positioned opposite the LEDs so as to detect the emitted light as it emerges from the tissue site. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.